Generally described, a combined cycle power generation system uses a combination of a gas turbine and a steam turbine to produce electrical power or otherwise to drive a load. Specifically, a gas turbine cycle may be operatively combined with a steam turbine cycle by way of a heat recovery steam generator and the like. The heat recovery steam generator may be a multi-section heat exchanger that allows feedwater for the steam generation process to be heated by the hot combustion gases of the gas turbine exhaust. The primary efficiency of the combined cycle system arrangement is the utilization of the otherwise “wasted” heat of the gas turbine exhaust. Power plant operators thus aim to generate the maximum possible useful work from the heat in the gas turbine exhaust.
Power plant operators are also interested in fuel flexibility. A conventional gas turbine combined cycle power plant, however, generally may be less efficient when operating on, for example, heavy fuel oil or fuels other than natural gas. The loss of efficiency due to the use of heavy fuel oils, however, may be limited by the addition of duct burning. Duct burning increases the exhaust temperatures so as to enable a supercritical steam cycle with steam at higher temperatures and higher pressures. Although the overall efficiency may improve, the high temperatures produced by such a supercritical cycle may be too high for the heat recovery steam generator to extract all the available energy from the hot exhaust gas stream passing therethrough.
Additional temperature limitations also may be imposed by the structure of the heat recovery steam generator itself. If certain temperatures are exceeded, expensive water cooled walls may become necessary. Because of this temperature limit, the amount of fuel that can be burned in a single duct burner stage may be limited. Placing the duct burner in a colder location may allow additional fuel to be burned but this positioning may expose the last sections of a high pressure superheater to unfired gas turbine exhaust. Moreover, if the turbine is producing relatively cold exhaust, this positioning may prevent a high pressure superheater section from producing steam at the desired temperature. This problem may be addressed by using multiple duct burner stages, but these additional stages add additional cost and complexity to the overall duct burner system. For example, a duct burner requires a significant length between the flame and the first superheater for a uniform flow of the combustion gases. Using a second row of duct burners would require additional heat recovery steam generator length as compared to a single burner duct design.
There is thus a desire for an improved duct fired combined cycle system with overall temperature controls. Such a system may allow the use of alternative fuels in an efficient manner but without requiring costly upgrades to the heat recovery steam generator and other components